Increasing the transmission capacity in optical networks requires demultiplexers, such as free space bulk gratings or integrated optics such as arrayed waveguide gratings (AWG) and echelle gratings, which can provide wider passband and lower ripple within the passband. This is particularly true for network systems transmitting higher bit rates which have wider spectra. Moreover, optical networks using reconfigurable optical add-drop modules (ROADM) have optical signals passing through cascades of demultiplexers and multiplexers, making accumulated losses more critical.
It is desirable that the passband of all the output channels is constant in a device, preferably having a same width and flat top profile. Numerous examples exist in the prior art of designs to increase the passband width, provide a flat top profile and to reduce ripple. However, it has been observed that the passband shape varies across the outputs in a standard AWG. The passband of the center output has the highest amplitude and frequency width which becomes lower and narrower for the outputs progressively further from the center. The passband variability leads to a deformation of the filter function from output to output reducing the flatness as well as increasing the ripple. This limits the performance of the AWG as it reduces the quality of the outer channels.
U.S. Pat. No. 6,768,842 issued to Alcatel Optonics UK Limited, Jul. 27, 2004 discloses an AWG in which the angle of the array waveguides at the star coupler waveguides is chirped to remove third-order aberration (COMA) which would otherwise cause asymmetry in the AWG output channel signals, especially where the AWG has a flattened passband. The shape of the star coupler is not changed.
It has been determined in accordance with the present invention that there is a phase linearity error between the grating line and the outputs on the focal line, comparing the center output to the surrounding outputs, when a confocal or Rowland configuration is used. This corresponds to a relative phase difference between each output which limits the passband. In the case of the AWG this degradation comes primarily from field aberrations in the output star coupler, whether confocal or Rowland. The same degradation happens also for the input star coupler if input waveguides are used away from the center.
U.S. Pat. No. 6,339,664 issued to British Technology Group, Jan. 15, 2002, discloses an AWG for which a non-linear Δ1 increment is designed to broaden the 3 dB passband. This is done by incorporating a non-linear, parabolic function in the path length increment of the waveguide array. It can be realized as a change in the grating line, or by altering the waveguides without changing the circular grating line. Either method changes the average phase. However, this patent does not recognize the phase error caused by the star coupler geometry. As a result no correction to individual output phase is made and the passband uniformity is not improved across the wavelength spectrum, even while the passband is broadened.
This phase linearity error results from a geometric configuration common to many optical gratings. For instance, a concave bulk optical grating or an echelle grating also typically include an arcuate grating line opposite an arcuate focal line, leading to the same phase error, which is generally ignored. Changing the phase in the array arriving at the grating line only changes the average phase seen by the outputs. It is the grating line geometry which controls output to output variations.
An object of the present invention is to provide an optical grating for which all of the outputs are substantially equal in amplitude and width.
It is a further object of the present invention to provide an optical grating for which all of the outputs are substantially phase matched.